1. Field of the Invention
The present invention relates to controlling the temperature of a substrate in a plasma reactor chamber. More specifically, the present invention provides for an apparatus and process for monitoring and sensing a change in temperature of a semiconductor substrate, and altering the temperature of the semiconductor substrate in accordance with the change of temperature.
2. Description of the Prior Art
Processing chambers for substrates often include a substrate support, such as a pedestal, for positioning the substrate in the chamber. The substrate could be a semiconductor wafer, a liquid crystal display, a plate of glass, a mirror, etc. The substrate support can be used to heat or cool the substrate. The process used in the chamber can be any desired process such as a chemical vapor deposition (CVD) or a plasma enhanced CVD (PECVD) process, for example. For a PECVD process, an RF field is applied between an electrode in the substrate support and the top of the chamber. A ceramic material may be applied to the top of the support to provide a dielectric to protect the substrate support electrode in a plasma process, or as part of an electrostatic chuck.
An electrostatic chuck is one type of apparatus for holding a wafer in place while it is being processed. Other methods include a vacuum applied to the bottom side of the wafer, or clamps for holding the wafer down. Clamps provide a non-uniform force and cover the edges of the wafer, while a vacuum applied to the bottom of the wafer is not as effective to hold the wafer in place when used in a chamber in which a vacuum is applied to the interior of the chamber, including the upper side of the wafer.
Electrostatic chucks are devices which have gained wide usage in the semiconductor industry for clamping semiconductor wafer during manufacturing processes, such as high density plasma reactions. Electrostatic chucks employ an electrostatic force between oppositely charged surfaces to secure the wafer to the chuck. Fabrication of some electrostatic chucks involves machining a process compatible metal, such as aluminum, into a suitable support pedestal and grit blasting the top surface of the pedestal. A layer of dielectric material, such as ceramic, is then plasma sprayed onto the upper surface of the pedestal and ground to a smooth, planar upper surface for supporting the wafer. Alternately, a polymer film, such as that sold under the trademark Kapton™, available from many well-known suppliers, may be used for the dielectric. During chemical processing of the substrate, the chuck functions by applying a high DC voltage between the chamber walls and the metal pedestal, causing positive charge on one side of the dielectric layer and negative charge on the other side. This charge generates an attractive, substantially uniform, coulomb force there between that secures the wafer to the dielectric layer. Alternately, multiple electrodes may be formed in the pedestal.
One important process parameter in reactor chambers for processing wafers is the temperature of the wafer. During processing, heat is often transferred to or from the substrate or wafer via surface conduction and/or convection between the substrate and the underlying substrate support or through an intervening backside gas. The temperature of the substrate support is typically regulated by circulating heat exchanging fluid such as water or gas, through channels within the substrate support. Alternately, resistive coils can be used to heat the substrate support. The substrate support can thus be used for heating or cooling the substrate. The efficiency of this method, however, is generally limited by the extent to which the backside of the substrate actually contacts the upper surface of the substrate support since, at the microscopic level, only small areas of the two surfaces actually contact each other. To facilitate heat transfer between the substrate and the substrate support, the regions between the contact points are typically filled with gas molecules, such as helium, argon, oxygen, or CF4 to enhance the thermal transfer between the substrate and the substrate support.
A number of prior art patents teach various methods for varying the heat transfer to or from different areas of a wafer. U.S. Pat. No. 4,502,094 teaches enhancing the thermal conductivity by having thermally conductive portions protrude from the susceptor beyond a dielectric layer on the susceptor. In particular, copper pillars are used to provide direct contact with the wafer and draw heat more rapidly. The thermally conductive protrusions are also electrically conductive. Another advantage of this design is that any small particles of debris which may be present on the chuck tend to be attracted onto the dielectric in the gaps between the pillars. Such a chuck is apparently useful in non-plasma reactors. In a plasma reactor, a dielectric is needed to prevent electrical shorts between a plasma electrode and the electrode in the electrostatic chuck, thus requiring the electrostatic chuck electrode to be covered with a dielectric.
U.S. Pat. No. 5,160,152 also discusses an approach using protrusions extending above the electrostatic chuck's surface. This patent distinguishes itself from a structure providing a gas underneath the wafer by providing projections on the top of the electrostatic chuck. U.S. Pat. No. 5,160,152 addresses wafers that get hotter in the middle, due to heat transfer through the sides of the chuck and due to a cooling jacket at the sides of the chuck. The area of the projections is made larger in the central portion of the wafer to provide more heat transfer at the center of the wafer. Smaller area projections are used in the periphery of the wafer. This patent relies on the direct conduction of heat through the projections themselves.
An alternate approach to heat transfer in an electrostatic chuck or other substrate support uses helium gas or another gas applied to the substrate support surface beneath the wafer. Ceramic dielectrics necessary for the electrostatic force on the top of an electrostatic chuck are not particularly efficient for heat transfer, both because of the limited heat transfer characteristics of ceramic material itself, and the inability to polish it sufficiently smoothly, thus leaving interstices at a microscopic level which preclude uniform contact. These interstices, however, can be advantageous. If a helium gas is applied through small holes in the interior of the substrate support, the helium gas can fill the space between the substrate and the wafer, to act as the heat transfer mechanism. One problem with such use of helium gas is preventing it from leaking from the periphery of the substrate support into the chamber itself. Another problem is to control the flow of helium gas to the substrate in such a manner that the wafer remains at a uniform temperature.
U.S. Pat. No. 5,761,023 to Lue et al., and assigned to the same assignee as the assignee of the inventions described herein and fully incorporated herein by reference thereto, teaches an improved substrate support and method for operating in which multiple pressure zones are provided on the surface of the substrate support. A seal area is provided between the different zones to allow different gas pressures in the different zones. A higher gas pressure is provided to a zone corresponding to an area of the substrate where greater heat transfer is desired. The gap between the substrate support and the wafer and the gas pressure are each selected to provide the desired amount of heat transfer. A feedback control loop is used to control the pressure in the different zones, and thus, control the temperature of the substrate. At least one temperature sensor is used to provide a temperature signal, with a controller responding to the signal to control the gas pressure to adjust the heating or cooling accordingly. U.S. Pat. No. 5,761,023 further reaches an electrostatic chuck having a dielectric whose thickness varies. In particular, the dielectric is made thicker in the middle of the support so that there is a greater electrostatic force around the periphery of the wafer. This improves the heat transfer at the periphery of the wafer both by virtue of preventing the edges from bowing due to a heat differential at the edges, and by enabling high pressure heat-transferring gas at the periphery of the wafer to be contained without escaping.
It is important that a change in temperature of any part of the wafer be sensed immediately because if different parts of the wafer are at different temperatures, structures on the wafer intended to be identical may be formed at different rates, thus producing inconsistent results. Accordingly, it is desirable to have a uniform temperature across the wafer. Therefore, what is needed and what has been invented is an improved apparatus and an improved process for controlling the temperature of a substrate in a plasma reactor chamber.